Grey background in screen capture

I have extended JToolTip to make fade-in effect for it, well it works nice but when tooltip is painted partly outside of main JFrame it uses HeavyWeightWindow created in javax.swing.Popup. And for that I have to get screen capture from region where this tooltip component lies.

I dont know what it is but when I create screen capture from exactly region where tooltip component lies the image is grey, now if I move the regions y coordinate lets say +10 the captured image is correct but +10 away from correct position.

Or now I noticed that screen capture wont work behind the javax.swing.Popup$HeavyWeightWindow window. ??? WHY NOT ???
How to fix this problem ???